The Great Step Forward

Going over some old thoughts the other day.  Pondering, as it were, the way jet engines have changed over the years.

No change in the basic theory, of course, but a lot of refinements and improvements in materials.

Of course, the manufacturers have put in a lot of effort to fine tune engines and improve gas flows and aerodynamics—especially in the turbine but there has been one thing that, to my mind, was the real turning point.  That moment in time when jet engines ‘grew up’.

Before I get into that, let’s just think about the ‘old days’ of  gas turbines.  Aero-gas turbines as opposed to hulking great industrial LP gas turbines with, or without, closed loop systems as made by the likes of Solar and ASEA Brown Boveri.

[NB: The Solar Saturn engine, first designed in 1950 for the US Navy and produced in 1960, went on to become the world's most widely used industrial gas turbine with some 4800 units in 80 countries. It remains in production today in two up-rated and enhanced configurations.]

Our good old ‘clunky’ engines were highly reliable and ruggedly built, for the most part.  They were pretty heavy for their thrust output but the idea was to contain all that heat and energy.

Of course, many of the old engines were centrifugal flow.  The wonderful sounding Derwent, Ghost, Goblin were all centrifugal flow as were the first turbopropellers—chief among them being the redoubtable Dart, still in use today.  First produced in the late 1940s, it powered the first Vickers Viscount maiden flight in 1948, and was still in production until the last F27s and H.S 748s were produced in 1987.

These centrifugal flow engines were very robust and had very simple systems; the fuel system on the Dart is a model of simplicity.

“Whittle… stressed the great simplicity of his engine. Hives [Director of Rolls Royce] commented, ‘We’ll soon design the bloody simplicity out of it.’ ” [From Genesis of the Jet]

How successfully they have achieved that!

[NB:  http://web.mit.edu/aeroastro/labs/gtl/early_GT_history.html

Is an interesting web page—especially the early history.]

Of course there were axial flow engines, too.  The Avon, Sapphire, Viper were all axial flow as were many of the early Pratt & Whitney engines.

Some engines were compounded.  The reverse flow Proteus turbopropeller engine had axial flow and a centrifugal last stage to ‘turn’ the air around the corner into the combustion section.  This is still something that is widely used, not least by the wonderful PT6 engine.

All these engines had alloy front ends with aluminised mild steel rear compressor sections and outer combustion casings.  The materials used were fairly standard in those days, the hot parts were various forms of nimonic—‘Hastalloy’, ‘Waspalloy’ (Registered Trade names of ‘Special Metals Group), which is a nickel alloy in various guises.

Materials have moved on.  We still use nimonics in conjunction with crystals for turbine blades but there is increasing use of lighter and stronger metals like titanium.  There is also more use being made of composites here and there on colder parts.

[NB: The ‘colder’ parts are not all ‘cold’.  Remember that the temperature of the gas coming off the compressor of a twin spool engine like a ‘Spey’ is around 550°C which is the same temperature as the glowing end of a cigarette when you suck on the other end.]

So where is the ‘breakthrough?  Is it the multi-spool engine like the JT9D or Spey that led to the Triple Spool RB211—a machine of great beauty and elegance?

Is it the development of high by-pass engines for the military airlift aeroplanes?

No, no.  None of these.

Lets go back to the Viper.

No, really!

The Viper was developed from the Adder engine that was developed, in turn, from the Mamba that came from a Metrovick project.  The twin Mamba was successfully installed in the Gannet and the Adder engine was the prototype Saab Draken engine.

‘Power by the Hour’ leasing was started in those days as the Viper had maintenance issues resulting from it being developed as a limited life engine (10 hours) for the Jindivik target drone.  Operators would pay a fixed hourly rate to Bristol Siddeley for the continual maintenance of the engines.

There was one major step forward.

But the main one was that these engines had vaporising burners.  Not a big issue.  Vaporising, hockey stick, burners were very efficient but hard to start so atomising burners (4) were used as well to start the engine and get the vaporising burners going.  The atomising burners would remain burning while the engine was running.

How is that a major leap forward?

Because they were in an annular combustion chamber.

Why is that so significant?

This engine, like the constant volume engine and the external combustion engine is a heat cycle machine.  It relies for its effectiveness and operation on the addition of heat energy.

Irrespective of the temperature of the gas coming off the compressor, fuel is burnt in the combustion chamber in order to add (heat) energy to the working fluid (air).

Separate combustion cylinders and can-annular (cannular) systems are limited to the number of burners they can hold and control—in terms of flame shape; this includes multiple burner systems on early Pratt & Whitney engines.

But annular chambers can have as many burners as you can fit in them.  This means you can add as much heat as the engine will take without a disproportionate increase in temperature.

Now, from that, we leap forward.  Old engines ran at 10:1 compression ratios.  It was thought that the maximum possible would be around 20:1 before all sorts of problems occurred.

Now we are running at over 40:1 compression ratio on a routine basis.  We have high by-pass engines where 80% of the airflow is going down the by-pass duct and doing most of the work and that means that the fan is being driven by 20% of the airflow in a small core engine.

Huge amounts of power are generated in the core turbine to drive the big fans which means that the pressure ratios must be higher, the temperatures are higher, the stresses are higher.

All this is possible because the amount of heat that can be added is exponentially increased by the use of annular combustion chambers

And that, dear friends, is all thanks to Bristol Siddeley and the humble, disposable, Viper engine.

By 'The Book'

Recently I have been speaking about safety.  I have said, time and again, to the point, no doubt, of boredom, that there is only one way to work on aeroplanes.

The right way.

Is 'The Book' always right?  How often have you read an entry in the Aircraft Maintenance Manual (AMM) and wondered if there could be a 'better way'?

Rarely, one hopes.  If you analyse 'your way' you may, very often, find that there is a flaw in your analysis or sequence of events for a particular task.

Sometimes events occur that highlight a flaw in the system.  A system that we have, hitherto, considered flawless.

The NTSB (National Transportation Safety Board in the United States) has written a letter to the FAA (Federal Aviation Administration) pointing out something that should have been obvious from the very start of commercial aviation and yet...

http://1.usa.gov/pLRZmM

This is very well worth reading, it will take a few minutes of your valuable time but make the effort.

We are not only concerned with what it is saying about this specific point but also we should consider the ramifications on everything else that we do.

Next time you work on an aeroplane  -  and by 'work' I am not just referring to maintenance people (wonderful souls though we undeniably are) but to flight crews, cabin crews, baggage handlers, catering delivery people, fuel tanker drivers, etc., and say to yourselves, "Am I missing something obvious?  Is what I am doing safe?  Is it safe for me, for the aeroplane, for other people?"

"IS IT SAFE?"

Flying is, inherently, safe.  Modern aircraft design and materials make it so.  Humans working on it reduce that level of security.

Working to 'The Book' should prevent that reduction.

Check.

Does it?

Addendum to "Think Safe"

In the last 'Blog' I have mentioned that there is a rule that every one of my students hears.  This is,

“Rule 1: Look after yourself”.

This is number 1 of “The Three Leyman Laws of Aviation Maintenance”

Let’s have a look at them now while they are fresh in the mind and applicable to the previous ‘Blog’.

Rule 1: Look after yourself.  Only you know what you are doing and why you are doing it.  You are the person best placed to observe and assess the risks and to take the appropriate action to avoid harm to yourself.

If you do not look after yourself then you are not going to be able to obey rule no.2.

Rule 2:  Look after your colleague.  It is eminently possible that the person, or persons, that you work with are not your friends.  It would be nice if this were always so but life being what it is you will find that, sometimes, you will work with someone you dislike.

Fact, you still have to work with them.

Fact, you are still in a team with them.

Fact, they are your colleagues.

Look after them because there may come a time when you need them to look after you.

Rule 3: Look after the equipment.  Much of the equipment used on modern aeroplanes to maintain them is extremely expensive.  Some of it is fairly delicate (ask your avionics people about the TDR – Time Domain Reflectometer!).  Certainly a lot of the equipment on the aeroplane is delicate and expensive – need I mention inertial navigational boxes?

If the equipment becomes defective it will require repair or replacing.  All airlines are pricing to the bone to be competitive, additional repair costs for equipment, especially Ground Servicing Equipment (GSE) could be the straw that broke the camel’s back.

At best you could be looking at a zero pay rise or, at worse, looking for a new job.

Possibly, defective or ill-maintained GSE could cause you injury (have you seen a toe that had a tow-bar - no pun intended, dropped on it?).

Either way, not looking after the equipment contravenes Rule 1 (see above).

Don’t say you weren’t warned!

Think 'SAFE'!

At great risk of sounding boring or repetitive, I should like to say, again, that when one is involved in aviation maintenance there is only one way to do things and that is the right way.

There are no ‘ifs and buts’, there is no negotiation or discussion necessary and there is no such thing as ‘it will do’!

Quite apart from the number of injuries and fatalities to passengers and crews due to aircraft making big black smoking holes in the ground, there are also risks involved with maintaining aeroplanes.

Time and time again I have said to students taking part in all sorts of courses, “Rule 1: Look after yourself.”

Sadly, this advice tends to be disregarded.  Primarily, I suspect, because most people believe that accidents happen to other people, they forget that, to everyone else you are the other person!

On my ‘Facebook’ page [http://on.fb.me/nDwHMO] I have given an example of what happens when the appropriate safety precautions are not taken.  There is also a mention there that two people a year, on average (I am told) are sucked into jet engine intakes.

The number of people who are struck and killed by propellers and tail rotors is significantly higher.

How does this happen?  Why does it happen?

For many years the makers of propellers and various Aviation Authorities – the CAA (UK) not least among them, have attempted to discover the magic paintwork that will show a propeller up even when it is spinning.

They have failed.  Without exception.

The fact is the propellers become invisible when they are rotating at any rpm that becomes inimical to human health (Translates: ‘lethal’).

I spend much time emphasizing to students that if they cannot see a propeller it is very dangerous.  Very.  This was even stated clearly in “A Simple Guide to Understanding Jet Engines” on Page 59 with a sad example given.

The air rushing into a gas turbine intake is also invisible.  There is a famous incident on a US aircraft carrier (USS Theodore Roosevelt, I do believe) where an airman was sucked into an A-6 Intruder intake and survived.  He survived because his helmet came off and smashed the compressor blades before his head arrived on the scene.

He was lucky.  Extremely lucky.  He was also foolhardy.

Propellers and air rushing into intakes are both invisible.  The movement of air from one place to another is unstoppable unless you are aerodynamically shaped to permit the flow of air to slip past.

Most people, however, tend to wear clothes when working.

“A Simple Guide to Understanding Jet Engines” Page 131, qv.

The primary cause of these accidents is somewhat deeper.  It is invisibility and the inability of the human person to fight the movement of air which creates the accident but there is something much more sinister that creates the situation!

Mindset.

We are, all of us, victims of compulsion and impulsion.  Every one of us has pressure applied to do the job quickly.  We understand that aeroplanes only make money when they are flying – on the ground they soak up money like a dry sponge.

There is constant pressure on us all to get the aeroplane back up in the air as quickly as possible, that there are schedules that must be kept.

Combine that with the idea that “familiarity breeds contempt”.

We adopt a laissez faire attitude, a feeling that ‘nothing ever happens’ or that the risk has been overstated (by people like me!).

But accidents happen.

“An Air New Zealand spokeswoman said the engine (RR-Allison T56) was sitting on a stand without propellors (sic) attached and was not affixed to a plane (C-130) at the time of the accident.”

A 51 year-old mechanic was sucked into the intake and died.

It is possible that he did not get ‘chopped up’ but his body, in blocking the intake, created a huge depression that damaged his lungs.  This must have been, necessarily, at low power because the propeller was not installed.

Low power - and yet...

This is a turbopropeller engine that develops around 6000HP.  Low power?  Still significantly higher than your Ford Focus engine!

It has something in common with a Ford Focus  -  it is NOT a toy!

Careless?  Yes.  Should he have known better?  Yes.

Ask yourself ‘how did it happen’?  Try to go through it in your mind.  Ghastly, isn’t it?

Most people believe that accidents happen to other people, they forget that, to everyone else you are the other person!